A crankshaft is normally used while it is installed in an engine. When the rotational imbalance occurs in the crankshaft, a variety of problems occur (e.g., generation of vibration in the engine revolution). Therefore, the amount of the rotational imbalance (hereinafter simply referred to as “an imbalance value”) of a crankshaft is required to be in a predetermined allowable value range. A position of center holes, used as a reference for the crankshaft working, is important for setting the imbalance value of the crankshaft to be within the allowable value range.
For example, a center hole is formed as follows. First, a balance center axis of a crankshaft (a material crankshaft) is found out. Specifically, a balance meter measures the crankshaft's balance while the material crankshaft is actually rotated. Then, center holes are bored in the both end surfaces of the crankshaft. Positions of the center holes are arranged on the obtained center axis. Furthermore, the balance meter measures balance about the center axis again in the final working phase. When imbalance is found out, the balance adjustment is executed by boring a hole in a counterweight.
When the crankshaft of a material state (i.e., material crankshaft) is finished in an ideal shape recorded in the design data, it is possible to easily set the imbalance value to be in the allowable value range by boring center holes in the center of a main journal. Even if balance disruption occurs in a subsequent working step, balance adjustment can be easily executed by finally boring a hole in the counterweight.
However, thickness of the material crankshaft could be actually formed unevenly due to a variety of factors including e.g., bad casting mold, bad forging mold, impact in trimming and mold stripping. In the working of the material crankshaft, most of the working steps are executed for a cylindrical part of a main journal and a cylindrical part of a pin journal. Therefore, the drawback of the uneven thickness is resolved for the respective cylindrical parts. However, working is hardly executed for the counterweight. Thickness of the counterweight therefore remains to be uneven. As a result, the quantity imbalance remains in the entire material crankshaft.
If the quantity imbalance greatly remains after the working, the imbalance value could not be set to be in the allowable value range, despite that the balance adjustment is executed by boring a hole in the counterweight. Additionally, when the balance adjustment cannot be completed within a predetermined cycle time, for instance, the imbalance value cannot be set to be in the allowable value range. This is because there is limitation for the number of holes allowed to be bored in the balance adjustment when the crankshaft is regarded as an inferior product.
Furthermore, the balance value could not be set to be in the allowable value range unless an essential portion is removed from the crankshaft.
When the imbalance value of the crankshaft cannot be finally set to be in the allowable value range as described above, modification is required for the working treatment. Specifically, the misalignment amount of the center-hole boring position is computed based on the balance measurement of the post-working crank shaft. The misalignment amount is fed back to the center-hole working processing. Then, modification is executed for a treatment in the center-hole working processing. In this case, such a material crankshaft is regarded as an inferior product that the center hole is bored before the feedback of the misalignment amount. Additionally, feedback of the misalignment amount is required every time the product lot of the material is changed. In this case, there is a drawback that a great number of processing steps are required.
Here, Laid-open Japan Patent Application Publication No. JP-A-H09-174382 discloses a method as a technology for deciding a position of center holes in a crankshaft. In the method, a plurality of materials is firstly extracted as samples. Then, regarding the respective extracted materials, a difference is computed between an axial center used as a reference in boring center holes and a position of the actually bored center hole, for instance. Subsequently, the correction amount in deciding the center-hole position is computed by statistically computing impact of the computed difference value on the imbalance amount after the finish working.
On the other hand, Laid-open Japan Patent Application Publication No. JP-A-S51-076682 discloses another technology for deciding a position of center holes. In this case, a dynamic balance point is obtained for the respective end surfaces of the material crankshaft by a dynamic balance test. Subsequently, a shape of a journal section in the material crankshaft is measured, for instance. Based on a result of the measurement, imbalance to occur after the working is arithmetically obtained. A center hole is then bored in an offset position, which is displaced from the dynamic balance point by the imbalance amount.
Note a method for measuring a shape of a workpiece and computing an axis of the workpiece has been known (see Laid-open Japan Patent Application Publication No. JP-A-2001-091244).